No option seen to export .3ds in 2.8
See the developer discussion about it here: https://developer.blender.org/T62576

It seems it is dropped officially. Personally, I still use it because sometimes that's the only format I can get models I need in, but while it's inconvenient, it is still possible to import 3ds to Blender 2.79 and copying models from 2.79 to 2.8 is no problem. As for exporting, there does not seem to be a direct way at the moment. I think the only convenient and straight forward option is to find another suitable format for your needs, and that should not be difficult. There is also a good chance someone will make an addon as 3ds is apparently still widely used and scripting the addon is probably not extremely difficult so it might just be a matter of time till it comes back.